Metal-working fluids have heretofore been inhibited from spoilage (degradation) by adding preservatives, particularly organic preservatives.
However, the spoilage-inhibiting effect of these organic preservatives varies depending on the type of microorganism; i.e., they show enough spoilage-inhibiting activity for specific microorganisms, but are insufficiently effective for the others. In using the organic preservatives, accordingly it is necessary to choose a suitable one taking into account the type of microorganism, and thus they lack general-purpose properties.
Some of the organic preservatives are decomposed by microorganisms. In the case of such organic preservatives, it is necessary for fresh preservatives to be added frequently since their spoilage-inhibiting effect is reduced in a short period of time. This gives rise to the problems that much labor and time are needed in the production of work pieces, and its production costs are increased. Furthermore most of the organic preservatives are sometimes harmful to human body, i.e., cause problems such as roughening of skin and rashes.
As another method of sterilization, it is known that metal-working fluid which has been used for a predetermined time is heated to relatively high temperatures to kill anaerobic bacteria in the metal-working fluid, and various additives lost by the heat treatment are supplemented simultaneously. Also in this method, however, much labor and time are needed, and furthermore no sufficient spoilage-inhibiting effect can be obtained.
A method has already been proposed in which metal-working fluid is contacted with metallic cobalt to inhibit the spoiling thereof (see Japanese Patent Application No. 22693/1982). This method, however, needs special equipment in the practice thereof.